1. Field of the Invention
The present invention generally relates to a communication-line-quality measuring system, and more particularly, to a communication-line-quality measuring system used in a two-way community antenna television (CATV) system and a personal handyphone system (PHS) in Japan, etc.
In the two-way CATV system, line quality between an office such as a switching office and terminal equipment on a subscriber side is measured. In the PHS, line quality of a radio transmission line between a base station and a mobile station (a portable station) on a subscriber side is measured.
In the line-quality measurement, it is desired that the measurement be carried out without influencing an operation of the communication system.
2. Description of the Related Art
FIG. 1 shows a typical network configuration of a two-way CATV telephone service. In FIG. 1, an office for the CATV system has a switching system 10, a plurality of TDMA (time division multiple access) equipment 11, 11a, 11b, 11c, which are connected to the switching system 10, and broadcasting equipment 1.
The office further includes a two-way combining-and-distributing unit 2 connected with the TDMA equipment 11, 11a-11c, and the broadcasting equipment 1, and an optical modulation-and-demodulation unit 3 connected to the two-way combining-and-distributing unit 2. The optical modulation-and-demodulation unit 3 is connected to an optical modulation-and-demodulation unit 5 on the subscriber side through an optical cable 4.
On the subscriber side, two-way amplifiers 7, 7a-7c, tap-off parts 8, 9, and terminal equipment 12, 12a in the subscriber houses are provided. The two-way amplifiers 7, 7a-7c are connected to the optical modulation-and-demodulation unit 5 using coaxial cables 6 in series or in a shunt manner. Further, each of the terminal equipment 12, 12a is connected to the optical modulation-and-demodulation unit 5 through the two-way amplifiers 7, 7a-7c and the tap-off parts 8, 9.
In such a configuration, down-link electrical signals produced from the broadcasting equipment 1 and the TDMA equipment 11, 11a-11c, which will be transmitted to the respective terminal equipment 12, 12a, are combined in the two-way combining-and-distributing unit 2. After that, the combined signal is converted to an optical signal in the optical modulation-and-demodulation unit 3, and is transmitted to the optical modulation-and-demodulation unit 5 through the optical cable 4.
The transmitted optical signal is converted into an electrical signal in the optical modulation-and-demodulation unit 5. After that, the electrical signal is amplified in the respective two-way amplifiers 7, 7a-7c through the coaxial cables 6. After the electrical signal is branched according to a connection configuration of the amplifiers, the branched electrical signals are transmitted to the respective terminal equipment 12, 12a through the TO parts 8, 9 having combining and distributing functions.
On the other hand, up-link electrical signals produced from the respective terminal equipment 12, 12a, which will be transmitted to the broadcasting equipment 1 and the TDMA equipment 11, 11a-11c, are transmitted to the optical modulation-and-demodulation unit 5 through the TO parts 8, 9 and the two-way amplifiers 7, 7a-7 while being combined.
In the optical modulation-and-demodulation unit 5, the combined up-link electrical signal is converted into an optical signal, and is transmitted to the optical modulation-and-demodulation unit 3 through the optical cable 4. In the optical modulation-and-demodulation unit 3, the optical signal is converted into an electrical signal.
The converted electrical signal is distributed in the two-way combining-and-distributing unit 2, and the distributed signals are transmitted to the respective TDMA equipment 11, 11a-11c and broadcasting equipment 1.
For providing a telephone service in such a network structure, signal processing between the TDMA equipment 11 and the terminal equipment 12 has an important role.
In the following, a description will be given of a prior-art signal processing between the TDMA equipment 11 and the terminal equipment 12, by referring to FIG. 2. FIG. 2 shows a detailed block diagram of internal configurations of the prior-art TDMA equipment 11 and the prior-art terminal equipment 12.
In FIG. 2, the TDMA equipment 11 is connected to the switching system 10 providing the two-way-CATV-system's telephone service, and the terminal equipment 12 provided in the subscriber's house is connected to a telephone 13. To simplify the description, the terminal equipment 12 and the TDMA equipment 11 are substantially directly connected to each other, and the optical transmission system such as optical modulation-and-demodulation units 3, 5 are not shown in FIG. 2.
In the following, a description will be given of a frame structure of the down-link signal transmitted from the TDMA equipment 11 to the terminal equipment 12 and a frame structure of the up-link signal transmitted in a reversal direction, by referring to FIG. 3 to FIG. 5C.
First, a description will be given of the frame structure of the down-link signal, by referring to FIG. 3. FIG. 3 shows the frame structure of the down-link signal represented in a matrix form. In the frame structure of the down-link signal shown in FIG. 3, in a row direction, one frame is constituted by 128 timeslots indicated by timeslot numbers (TSNo.) "0" to "127". The each frame has a 125-.mu.sec time interval. One timeslot is constructed with 8 bits.
In a column direction, a plurality of frames indicated by frame numbers (frame No.) "00" to "31" are arranged in the order of the frame numbers to constitute one multiframe. Since the multiframe is constructed with 32 frames, the one multiframe has a 4-msec time interval.
When the down-link signal having such a multiframe structure is transmitted from the TDMA equipment 11 to the terminal equipment 12, the frames "00" to "31" are successively transmitted in that order, and, after that, the frames "00" to "31" of the next multiframe are successively transmitted.
A symbol "MF" in the frame "00" is called a multiframe bit, and is constructed with a bit pattern indicating a top position of the multiframe. A symbol "F" in the respective frames "01" to "31" is called a frame bit, and is constructed with a bit pattern indicating a top position of the respective frame.
In the timeslots "02" to "32" in the respective frames "00" to "07", M channels (1) to (8) are provided. The M channels (1) to (8) are commonly used for measuring a distance between the TDMA equipment 11 and the terminal equipment 12 by measuring a delay time of the up-link signal returned from the terminal equipment 12 on the subscriber side to the TDMA equipment 11.
The TDMA equipment 11 sends measured distance information to the terminal equipment 12, and controls a transmission timing from the terminal equipment 12 to the TDMA equipment 11. In this way, a transmit signal from the terminal equipment 12 is synchronized with a system clock signal in the TDMA equipment 11 for each bit.
In the timeslots "02" to "32" in the respective frames "08" to "31", D channels (1) to (24) are provided. The D channels (1) to (8) are used as control channels for controlling the terminal equipment 12 on the subscriber side. For example, the D channel is used for transmitting a variety of information such as calling, terminating, and informing of an available B channel (discussed later).
The B channels B0 to B94 provided in the timeslots "33" to "127" of the respective frames are channels for speech and data, and are used for transmitting, for example, speech of a person using the telephone 13 connected to the terminal equipment 12 in the subscriber's house, and data of a personal computer connected to the terminal equipment 12.
FIG. 4 shows illustrations for explaining a relationship between the frame structure of the up-link signal and the frame structure of the down-link signal. An illustration (a) shows the one multiframe in which the frames "00" to "31" are arranged in series. An illustration (b) shows a configuration of the frame "00", and an illustration (c) shows a configuration of the frame "31". Further, an illustration (d) shows a configuration of the B channel B0 in the frame. As shown in the illustration (d), each B channel is constructed with 8 bits of b0 to b7, and has a 975-nsec time interval.
On the other hand, an illustration (e) shows the frame structure of the up-link signal. The frame structure of the up-link signal is constructed with the M channel, a distance-measurement window, the D channel, and the B channels B0 to B94. The time interval of the whole up-link signal frame is 4 msec, which is the same as that of the down-link signal frame.
The M channel and the distance-measurement window of the up-link signal are, in the same way as the down-link signal, used for measuring the distance between the TDMA equipment 11 and the terminal equipment 12 by measuring a delay time of the up-link signal returned from the terminal equipment 12 in the subscriber to the TDMA equipment 11.
The TDMA equipment 11 informs measured distance information to the terminal equipment 12, and controls a transmission timing from the terminal equipment 12 to the TDMA equipment 11. In this way, a transmit signal from the terminal equipment 12 is synchronized with a clock signal in the TDMA equipment 11 for each bit.
FIG. 5A shows a frame structure of the M channel. The frame structure of the M channel is constructed with a 16-bit guard timing (G), a preamble, a unique word (UW), data, and a cyclic redundancy check (CRC). The preamble and the unique word are formed by 256 bits, and the data and the CRC are formed by 64 bits.
The guard timing (G) is provided as a buffer time so as to have no influence on the previous and next channels even if the terminal equipment 12 misses the transmission timing by several bits. In the terminal equipment 12, during the guard timing, transmission of a carrier is also stopped.
The preamble is provided such that the TDMA equipment 11 synchronizes with a signal transmitted from the terminal equipment 12. In general, in the preamble data, data "0011" is repeatedly provided. The unique word is provided for indicating an end position of the preamble or a start position of the data. After the TDMA equipment 11 detects the preamble, data reception in the TDMA equipment 11 starts.
The CRC is provided for detecting a data error. In the TDMA equipment 11, when the CRC is checked and a data error is detected, the erroneous data is processed as invalid data. In this case, the terminal equipment 12 is controlled to transmit the same data again.
The D channel shown in the illustration (e) of the FIG. 4 is, in the same way as the down-link signal, a control channel for controlling the terminal equipment 12 on the subscriber side. For example, the D channel is used for transmitting a variety of information such as calling, terminating, and informing of an available B channel.
FIG. 5B shows a frame structure of the D channel. The frame structure of the D channel is constructed with the guard timing (G), the unique word (UW), the data, and the CRC. The guard timing and the unique word are formed by 24 bits, and the data and the CRC are formed by 256 bits.
On the side of the terminal equipment 12, synchronization can be established by the M channel. Therefore, the D channel has no preamble as compared to the M channel.
An illustration (f) in FIG. 4 shows a frame structure of each B channel in the up-link signal represented by (e) of FIG. 4. The frame structure of each B channel has a 34-.mu.sec time interval, and is constructed with the guard timing (G), the unique word (UW), and data of b0 to b255.
FIG. 5C shows a frame structure of the B channel. The B channel is constructed with the guard timing (G), the unique word (UW), and the data. The guard timing and the unique word are formed by 24 bits, and the data is formed by 256 bits. The B channel has no preamble for the same reason as that of the D channel. Further, the B channel has no CRC, because it is difficult to process the CRC in the B channel. In general, a re-transmission request due to a communication error in the B channel depends on the terminal equipment 12.
In the following, a description will be given of the two-way CATV system supplying the telephone service which is provided by the abovediscussed frame structures of the down-link signal and the up-link signal, by referring to FIG. 2.
Returning to FIG. 2, the TDMA equipment 11 includes interface parts (PRI parts) 20 to 23 for interfacing with the switching system 10, a network part (NW part) 24, a D1-channel interface part (D1 part) 25, a CPU 26, a frame assembling part (FRMA part) 27, a frame disassembling part (FRMD part) 28, M-channel interface parts (M parts) 29, 30, D2-channel interface parts (D2 parts) 31, 32, a modulation part (MOD part) 33, and a demodulation part (DEM part) 34.
A numeral "35" indicates a wave distributing part (WD part) included in the two-way combining-and-distributing unit 2 shown in FIG. 1. The WD part 35 is connected to the MOD part 33 and the DEM part 34. Further, a maintenance console part (MC part) 36 is connected to the CPU 26.
The terminal equipment 12 includes a WD part 38, a DEM part 39, a MOD part 40, a FRMD part 41, a FRMA part 42, M parts 43, 44, D2 parts 45, 46, B-channel control parts (BC parts) 47, 48, a CPU 49, and a subscriber-line interface part (SLI part) 50. Further, the telephone 13 is connected to the SLI part 50.
In the above-discussed structure, the PRI parts 20 to 23, connected between the switching system 10 and the NW part 24, are provided by trunk cards in an integrated services digital network (ISDN), and have an interface function between the switching system 10 and the NW part 24.
The NW part 24 is connected between the PRI parts 20 to 23 and the FRMA part 27, and is also connected between the PRI parts 20 to 23 and the FRMD part 28. Further, the NW part 24 is provided in a path between the PRI parts 20 to 23 and the CPU 26 through the D1 part 25. The NW part 24 has a speech data exchange processing function between the PRI parts 20 to 23 and any one of the FRMA part 27 and FRMD part 28. Further, the NW part 24 has a D1-channel interface function by the D1 part 25 provided between the PRI parts 20 to 23 and the CPU 26. The D1 channel is a call-processing control channel between the switching system 10 and the TDMA equipment 11.
The FRMA part 27 is connected to the NW part 24, the MOD part 33, and the CPU 26 through the M part 29 and the D2 part 31. The FRMA part 27 has a function of assembling speech data (64 kbps/channel.times.95 channels) transmitted from the 4 PRI parts 20 to 23 on the B channels B0 to B94 through the NW part 24 and control signals transmitted from the CPU 26 on the M channels and the D channels through the M part 29 and the D2 part 31, to a 8.192-Mbps (8 bits.times.128 timeslots.times.32 frames/4 msec) frame shown in FIG. 3.
The MOD part 33 is connected to the WD part 35, modulates a digital signal transmitted from the FRMA part 27 by means of a quadrature phase shift keying (QPSK), and converts the modulated digital signal into an RF (radio frequency) signal. The RF signal is transmitted to the terminal equipment 12 through the WD part 35.
FIG. 6 shows frequency allocation of the down-link RF signal toward the terminal equipment 12 and the up-link RF signal toward the TDMA equipment 11. In FIG. 6, as shown by a numeral "52", an overall frequency band of the up-link signal ranges from 10 to 50 MHz, and any 6-MHz band of the overall frequency band 52 is allocated to the up-link RF signal, as indicated by a numeral "53".
With regard to the down-link signal, as shown by a numeral "54", an overall frequency band of the down-link signal ranges from 70 to 550 MHz, and any 6-MHz band of the overall frequency band 54 is allocated to the down-link RF signal, as indicated by a numeral "55".
On the other hand, the up-link RF signal transmitted from the terminal equipment 12 is transmitted to the DEM part 34 through the WD part 35, and is converted down. The down-converted up-link signal is demodulated by means of the QPSK, and is provided to the FRMD part 28. In this case, the signal frame is constructed with the M channel, the D channel, and the B channel shown in FIG. 5.
The FRMD part 28 is connected to the NW part 24, and is connected to the CPU 26 through the M part 30 and the D2 part 32. The FRMD part 28 disassembles the signal frame produced from the DEM part 34 to the M channel, the D channel, and the 95 B channels.
The data on the B channels is transmitted to the PRI parts 20 to 23 through the NW part 24, and the data on the M channel and the D channel is transmitted to the CPU 26 through the M part 30 and the D2 part 32.
The CPU 26 processes a call control between the switching system 10 and the TDMA equipment 11 by using the D1 channel, a distance control between the TDMA equipment 11 and the terminal equipment 12 by using the M channel, a call control by using the D2 channel, and a maintenance control by means of an operation of the MC part 36. Namely, the MC part 36 is used for monitoring a condition of the terminal equipment 12.
The WD parts 35, 38 are connected to each other through a connecting transmission line between the TDMA equipment 11 and the terminal equipment 12. Each of the WD parts 35, 38 has a two-way band-pass filtering function of passing both the up-link RF signal and the down-link RF signal.
The WD part 35 has functions of combining the down-link RF signal path from the MOD part 33 and the up-link RF signal path to the DEM part 34, and combining a plurality of signals from the TDMA equipment 11, 11a-11c. The WD part 38 has a function of combining the up-link RF signal path from the MOD part 40 and the down-link RF signal path to the DEM part 39. The WD part 38 is provided within the terminal equipment 12.
The DEM part 39 in the terminal equipment 12 is connected between the WD part 38 and the FRMD part 41. The DEM part 39 converts the down-link RF signal produced from the WD part 38 to a lower frequency signal, and demodulates the signal by means of QPSK. A demodulated digital signal, which has the frame structure shown in FIG. 3, is transmitted to the FRMD part 41.
The FRMD part 41 is connected to the CPU 49 through the M part 43, the D2 part 45, and the BC part 47, and is also connected to the SLI part 50. The FRMD part 41 derives an M-channel signal and a D2-channel signal from the down-link signal produced from the DEM part 39, and transmits these derived signals to the CPU 49 through the M part 43 and the D2 part 45. Further, from a control of the BC part 47, the FRMD part 41 transmits the B-channel data designated by the CPU 49 to the SLI part 50.
The SLI part 50 has a CODEC (coder and decoder) function and a telephone-interface function. The SLI part 50 converts the B-channel data transmitted from the FRMD part 41 into an analog signal by the CODEC function, and transmits the analog signal to the telephone 13. Further, the SLI part 50 converts an analog signal transmitted from the telephone 13 into a digital signal by the CODEC function, and transmits the digital signal to the FRMA part 42.
The FRMA part 42 is connected to the CPU 49 through the M part 44, the D2 part 46, and the BC part 48, and, also, is connected to the MOD part 40. When an instruction for transmitting the B channel is provided from the CPU 49 through the BC part 48, the B-channel data transmitted from the SLI part 50 is provided to one timeslot of the B channels "B0" to "B94" in the up-link frame (e) shown in FIG. 4, and is transmitted to the MOD part 40.
Further, when an instruction for transmission of the M channel or the D channel is provided from the CPU 49, the M-channel data or the D-channel data is provided to the respective time slots, and are transmitted to the MOD part 40.
The MOD part 40 is connected to the WD part 38, and, only when the data is transmitted from the FRMA part 42, modulates the data by means of QPSK. Further, the MOD part 40 converts the modulated data into a higher frequency up-link RF signal, and transmits the up-link RF signal to the TDMA equipment 11 through the WD part 38.
The CPU 49 has a distance control function by means of the M channel, a call control function by means of the D2 channel, and a function of controlling the SLI part 50.
In the above-discussed two-way CATV supplying the telephone service, to confirm whether the information is correctly transmitted between the TDMA equipment 11 and the terminal equipment 12, a line-quality measuring unit may be provided.
In the line-quality measuring unit, by measuring data-transmission errors between the TDMA equipment 11 and the terminal equipment 12, the line quality may be detected. In general, when the measured line quality is less than a predetermined value, a cause why the line quality is degraded is analyzed, and a countermeasure is considered. In the following, typical causes of line-quality degradation will be described.
When connection in the connectors of the coaxial cables as transmission lines is poor, outside noise may enter into the transmission lines. When a malfunction occurs, such as, for example, a sealing material of the coaxial cable breaking, outside noise likewise enter into the transmission lines. Further, when amplifiers provided at intervals of 200 m in the transmission line malfunction, noise may also occur.
Next, a description will be given of a prior-art line-quality measuring method, by referring to FIG. 7 and FIG. 8. FIG. 7 shows a block diagram of the prior-art line-quality measuring system for the down-link signal. FIG. 8 shows a block diagram of the prior-art line-quality measuring system for the up-link signal. Elements in FIG. 2 which are the same as those of FIG. 7 and FIG. 8 are given the same reference numerals. Further, to simplify the description, descriptions for the same elements will be eliminated.
In FIG. 7, in the TDMA equipment 11, an interface part (INF part) 57 is provided instead of the FRMA part 27 shown in FIG. 2, and a conventional line-quality measuring unit (MEAS part) 58 is connected to the INF part 57.
Further, in the terminal equipment 12, an interface part (INF part) 59 is provided instead of the FRMD part 41 shown in FIG. 2. By connecting a line-quality measuring unit (MEAS part) 60 to the INF part 59, the down-link line-quality measuring system is constructed.
In the down-link line-quality measuring system, the PRI parts 20 to 23, the NW part 24, and the CPU 26 in the TDMA equipment 11 shown in FIG. 2 are not used. The INF part 57 is provided between the MEAS part 58 and the MOD part 33, and is operative only as interface means transiting a signal produced from the MEAS part 58 to the MOD part 33.
Further, the INF part 59 in the terminal equipment 12 is provided between the DEM part 39 and the MEAS part 60, and is operative only as interface means transiting a signal produced from the DEM part 39 to the MEAS part 58.
In such a configuration, a pseudo-random noise signal is successively produced from the MEAS part 58 in the TDMA equipment 11, and is transmitted to the terminal equipment 12 through the INF part 57, the MOD part 33, the WD part 35, and the transmission line. The pseudo-random noise signal is input to the DEM part 39 through the WD part 38, and is demodulated. The demodulated pseudo-random noise signal is transmitted to the MEAS part 60 through the INF part 59.
The MEAS part 60 can generate a reference signal which is the same as the pseudo-random noise signal generated in the MEAS part 58. Therefore, the MEAS part 60 compares the pseudo-random noise signal transmitted from the TDMA equipment 11 with the reference signal, and measures a bit error rate (BER) in the down-link communication line from the TDMA equipment 11 to the terminal equipment 12. When a value of the BER is larger than a predetermined value, it is determined that the down-link communication line is in a poor condition.
On the other hand, in FIG. 8, in the TDMA equipment 11, an interface part (INF part) 62 is provided instead of the FRMD part 28 shown in FIG. 2, and a conventional line-quality measuring unit (MEAS part) 63 is connected to the INF part 62.
Further, in the terminal equipment 12, an interface part (INF part) 64 is provided instead of the FRMA part 42 shown in FIG. 2. By connecting a line-quality measuring unit (MEAS part) 65 to the INF part 64, the up-link line-quality measuring system is constructed.
In such a configuration, a pseudo-random noise signal is successively produced from the MEAS part 65 in the terminal equipment 12, and is transmitted to the TDMA equipment 11 through the INF part 64, the MOD part 40, the WD parts 38, 35, and the transmission line. The pseudo-random noise signal is input to the DEM part 34, and is demodulated. The demodulated pseudo-random noise signal is transmitted to the MEAS part 63 through the INF part 62.
In the same way as the MEAS part 60, the MEAS part 63 can generate a reference signal which is the same as the pseudo-random noise signal generated in the MEAS part 65. Therefore, the MEAS part 63 compares the pseudo-random noise signal transmitted from the terminal equipment 12 with the reference signal, and measures a bit error rate (BER) in the up-link communication line from the terminal equipment 12 to the TDMA equipment 11. When a value of the BER is larger than a predetermined value, it is determined that the up-link communication line is in a poor condition.
The above-discussed communication-line-quality measuring system is applicable to the personal handyphone system (PHS) which is in commercial use in Japan. FIG. 9 shows a configuration example of the PHS. The PHS shown in FIG. 9 is constructed with a switching office 67 connected to a telephone network (not shown), a base station 68 connected to the switching office 67 through a transmission cable, and a subscriber's mobile terminal 69 carrying a radio communication with the base station 68 through a radio transmission line.
In such a system, the above-discussed TDMA equipment 11 is provided in the switching office 67, and the above-discussed terminal equipment 12 is provided in the mobile terminal 69. When line quality of the radio transmission line is measured, line-quality measurement is carried out by constructing the communication-line-quality measuring systems shown in FIG. 7 and FIG. 8.
However, there is the following problems in line-quality measurement by means of the communication-line-quality measuring systems shown in FIG. 7 and FIG. 8.
When the communication-line-quality measuring system is set up, an expert must go to the subscriber's house where the terminal equipment 12 is provided in order to provide a measurement apparatus (MEAS parts 60, 65) to the terminal equipment 12. Therefore, it takes much time and high cost.
Further, in the prior-art line-quality measuring system, it is substantially difficult to measure line quality of the terminal equipments 12 in all the subscribers.
During measurement, the telephone services for all the subscribers connected to the TDMA equipment 11 must be stopped.
In general, the data according to the frame structure shown in FIG. 3 and FIG. 5 is transmitted through the transmission line. However, when line quality is measured by means of the prior-art line-quality measuring system, the pseudo-random noise signal is successively transmitted regardless of the frame structure. Therefore, the signal structure in line-quality measurement is different from the practically-used signal structure in a general communication, and, thus, a precise line-quality measurement may not be carried out.